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1 Introduction organisms (Woolf et al. 2010; Wu et al. 2021a). Many Biochar is a pyrogenic carbon-rich material formed processes can be used to produce biochar such as slow through the thermal decomposition of biomass feed- and fast pyrolysis, microwave-assisted pyrolysis, hydro- stock under an oxygen-limited condition (Hagemann thermal carbonization, gasification, flash carboniza - et al. 2017; Inyang et al. 2015; Woolf et al. 2021). It tion, and torrefaction (Kostas et al. 2020; Wu et al. has been established that different biomass materials 2020; Wu et al. 2021a; Xiao et al. 2018) (Fig. 1). Slow have been utilized to produce biochar which entails pyrolysis and hydrothermal carbonization are the most agricultural waste, forestry waste, garden waste, food frequently used methods for the preparation of bio- waste, livestock manure, sewage sludge, and aquatic char (Dutta et al. 2021; Lian and Xing 2017). Biochar Wu et al. Biochar (2023) 5:6 Page 3 of 21 is a heterogeneous mixture containing both amorphous Meanwhile, the improvement in soil functions after less-carbonized fractions and microcrystalline graph- biochar application is beneficial to alleviate the salinity ite-like aromatic structures. It has special properties, and drought stress on plants (Mansoor et al. 2021; Zhu such as highly developed porosity, high specific surface et al. 2020). Finally, the multifunctional characteristic of area (SSA), abundant surface functional groups, and biochar makes it an effective material for environmental high content of mineral components. Biochar proper- pollution (Ahmad et al. 2014; Nidheesh et al. 2021). The ties are mainly regulated by feedstock composition and biochar’s multiple functions are connected to its physico- preparation procedures (Ahmad et al. 2014; Cha et al. chemical properties (Manya 2012; Zhang et al. 2021g). 2016; Lian and Xing 2017). Considering the striking heterogeneity in physicochemi- Biochar has received increasing attention for its poten- cal properties of biochar, an in-depth understanding of tial benefits in carbon sequestration, climate change miti - the developments of biochar regarding its multifunc- gation, waste management, bioenergy production, soil tional applications is urgently required. improvement, and pollution control due to its unique Although several meta-analyses and review studies properties (Nidheesh et al. 2021; Xiao et al. 2018; Yang have been carried out to evaluate biochar’s multifunc- et al. 2021e). Biochar has a high recalcitrant carbon (C) tional applications systematically, very few have investi- content and the strong adsorption capacity to carbon gated current trends and scientific developments in this dioxide (CO ). Its sustainable production and field appli - field. A conclusive study was performed here and could cations enable its great potential for long-term carbon help us to identify the main subject fields, the current storage and carbon emission reduction, thereby mitigat- research frontiers, and hotspots of biochar research ing climate change (Feng et al. 2021b; Yang et al. 2021e; (Wu et al. 2019; Wu et al. 2021a). Because new biochar Zhang et al. 2022). The conversion of biowaste, especially applications emerge, the main signs of progress and sewage sludge, into biochar through pyrolysis provides insights associated with this field will change over time. an alternative and promising way of managing waste bio- Additionally, the unintended consequences of biochar mass (Chen et al. 2020). The produced biochar can act as application are likely to emerge after long-term bio- a catalyst for bioenergy production (Malyan et al. 2021). char application. Therefore, it is necessary to provide Again, biochar positively affects plant productivity and an analytical overview of the main signs of progress and crop yields by improving soil quality and fertility levels insights into biochar research in 2021. (Farkas et al. 2020). Fig. 1 Biochar production and modification. Modified from Wu et al. (2020) Wu et al. Biochar (2023) 5:6 Page 4 of 21 Bibliometric analysis proceeded by Citespace software sharply, and this growth trend will continue, which indi- has gained rising interest in many fields due to its math - cates the huge and increasing attention as well as the ematical statistics and visual analytic functions (Chen increased amount of scientific knowledge of biochar 2004, 2017; Ren et al. 2021). It is believed to be an effec - research. tive tool for quantitatively evaluating a given field’s cur - rent situation and emerging trends (Kamali et al. 2020). 3.2 Contributing country analysis In this study, a combined bibliometric analysis and criti- As can be realized from Additional file 1: Fig. S2 and cal appraisal of the related documents were utilized to Table S1, China is the highest contributing country, quantitatively identify the research status and trend of accounting for 43.99% of the total publications. This is biochar in 2021. The aims of this review are to: (1) ana - most likely because China is a great agricultural country lyze the primary authors, countries, and keyword fre- with a large population, where agriculture occupies a sig- quency and hotspot trend of the publications; (2) identify nificant and strategic position in the national economy. the current trends and hot topics in the biochar field; (3) China was followed by the USA, with 504 publications, provide perspectives on full-scale applications of biochar. accounting for 9.11%. Besides China and USA, India, Paki- stan, South Korea, Australia, Canada, Brazil, Saudi Arabia, 2 Methods and Germany also have many scientific documents. Many 2.1 Literature search connections between countries indicate robust global A systematic literature search was conducted from the cooperation and communication in this field. Web of Science (WoS) Core Collection database using the keyword of "Biochar" in the title, keywords, and 3.3 Research institution analysis abstract (Ren et al. 2021). A total of 5533 documents The top 10 institutions with the most publications in the were collected from the database in 2021. To guarantee field of biochar research are summarized in Additional the validity of data collected, the synonyms associated file 1: Table S2. Chinese Academy of Sciences published with biochar research, such as “Cadmium” and “Cd”, “car- the largest number of papers in this field, followed by the bon dioxide” and “CO ”, “anaerobic digestion” and “AD”, University of Chinese Academy of Sciences and Zhejiang “nanoscale zero-valent iron” and “nZVI”, and “polycy- University. The Chinese Academy of Sciences is expected clic aromatic hydrocarbon” and “PAHs” were manually to be the leading institution in many fields. It can also be merged. found that 8 institutions were from China among the top 10 productive institutions. 2.2 S cientometrics analysis tools Cite-space is a new statistical analysis tool widely used 3.4 Keyword analysis for bibliometric analysis and visualization (Li et al. 2020; 3.4.1 Bio char for toxic metal immobilization Ren et al. 2021). It is a Java-based software first developed Biochar utilized for immobilizing toxic metals has caused by Professor Chaomei Chen and his team in 2004 (Chen the interest of researchers in 2021 (Fig. 2). Keyword co- 2004). Relevant documents were exported from the occurrence analysis suggested that the most studied toxic “marked list” (with “plain text” format) of WoS Core Col- metal was Cd (Table 1). Such heightened research inter lection and then inserted in Cite-space (5.3.R4) to ana- est in Cd was due to its being highly toxic and carcino- lyze their specific characteristics. The visual analysis of genic even at low concentrations (Purkayastha et al. 2014; cooperation networks of authors, contributing countries, Rai et al. 2019). institutions, categories, and keywords helps to reveal the The transformation of sewage sludge to biochar via clustering, knowledge structure, and emerging trends of pyrolysis provides a feasible approach for the safe dis biochar research. In the co-occurrence network maps, posal of the sludge (Buss 2021; Zhang et al. 2021f ). The each node corresponds to one item (e.g., author, institu- sewage sludge-derived biochar had a much lower leach- tion, keyword), and lines link the nodes. The higher the ing potential of heavy metals than the sewage sludge itself node’s frequency, the larger the node’s size. The thickness (Wang et al. 2021d; Xiong et al. 2021). Recently, co-pyrol- of the line indicates the cooperation degree between the ysis of sewage sludge and waste biomass (e.g., rice straw, two nodes. rice husk, cotton stalk, and sawdust) has been proven to improve the porous structure and reduce the leaching 3 Results and discussion toxicity of heavy metals in sewage sludge-derived biochar 3.1 P ublications about biochar research (Tong et al. 2021; Wang et al. 2021d, 2021f, 2021g; Xiong A total of 5533 documents were extracted from WoS et al. 2021). Another study also reported that co-pyrolysis Core Collection in 2021 (Additional file 1: Fig. S1). It can of sewage sludge and calcium sulfate was conducive to be seen that the annual number of publications increased Wu et al. Biochar (2023) 5:6 Page 5 of 21 Fig. 2 Keyword co-occurrence map of biochar research in 2021 Table 1 The top 20 keywords related to biochar research in 2021 shown promising results. The abundant mineral constitu - ents in sewage sludge-derived biochar, such as Si, Al, Fe, Rank Keyword Frequency and Ca, greatly participated in cationic metal removal via 1 Biochar 2774 cation exchange, complexation, and precipitation (Gopi- 2 Adsorption 1064 nath et al. 2021; Islam et al. 2021; Jellali et al. 2021). 3 Removal 716 Engineered biochar has gained much attention in terms 4 Pyrolysis 668 of immobilizing toxic metals in 2021. Modification may 5 Heavy metal 605 improve the carbon content, SSA, porous structure, 6 Aqueous solution 573 functional groups, and stability of biochar. Recently, 7 Bioma 545 metal oxides and salts have been widely used for biochar 8 Carbon 486 modification. Magnetic or metal-based biochar syn - 9 Activated carbon 484 thesized by using the transition metals (e.g., Fe, Co, Ni) 10 Sorption 449 and their oxides (e.g., F e O and Fe O ) with waste bio- 2 3 3 4 11 Water 448 mass exhibits advantages over pristine biochar in heavy 12 Soil 439 metal immobilization. For instance, a novel functional 13 Mechanism 419 colloid-like magnetic biochar with high dispersibility 14 Waste 327 synthesized via impregnation of Fe(II)/Fe(III)/artificial 15 Cadmium 325 humic acid onto corn stalk-derived biochar followed by 16 Pyrolysis temperature 312 torrefaction activation exhibited superior removal per- 17 Performance 311 formance for Cd (the maximum adsorption capacity of −1 18 Sewage sludge 303170 mg g ) (Yang et al. 2021c). The removal mechanisms 19 Waste water 297 included ion exchange, Cd–π interaction, complexation, 20 Temperature 294 and precipitation (Fig. 3a). Hierarchically porous mag- netic biochar produced from K FeO pre-treated wheat 2 4 −1 straw at 700 °C effectively removed Cd by 80 mg g (Fu immobilizing heavy metals in sewage sludge-derived bio- et al. 2021b). The pre- and post-heat treatment modi - char (Liu et al. 2021a). The application of sewage sludge- fication with Fe could produce Fe-loaded biochar with derived biochar as a sorbent for toxic metal removal has different properties. Generally, the Fe pre-modification Wu et al. Biochar (2023) 5:6 Page 6 of 21 Fig. 3 Possible pathways for the removal of toxic metals by biochar. Cd removal mechanisms by functional colloid-like magnetic biochar (a) (Yang et al. 2021c); Hg(II) removal mechanisms by sulfurized magnetic biochar (b) (Hsu et al. 2021); As(III) removal mechanisms by M nO -biochar composite (c) (Cuong et al. 2021) Wu et al. Biochar (2023) 5:6 Page 7 of 21 loaded greater Fe O micro-/nanoparticles on biochar Although most of the positive results are achieved for 3 4 and achieved greater surface area, while the Fe post- acidic heavy metal-contaminated soil amended with bio- modification enriched oxygen-containing functional char, there needs to be more research on the remediation groups on biochar (Reynel-Ávila et al. 2021; Zhao et al. effect of biochar on alkaline heavy metal-contaminated 2021b). Hsu et al. (2021) synthesized sulfurized magnetic soil. Previous studies showed that conventional biochar biochar via a single heating step. The product had high showed no significant remediation effect in alkaline soil. adsorption for Hg(II) (the maximum adsorption capacity Meanwhile, biochar amendment might reduce soil fertil- −1 of 8.93 mg g ), and C-S functional groups were the vital ity and cause soil alkalization. Fe- and Fe/Zn-modified adsorptive sites for Hg(II) (Fig. 3b). biochar application promoted the transformation of For metal anions (mainly As and Cr), biochar-based exchangeable Cd into oxidizable and residual Cd (Sun composites are effective sorbents for Cr and As. An et al. 2021c; Yang et al. 2021g). Moreover, Fe-modified active MnO -biochar composite was prepared to enhance biochar increased the richness and diversity of bacterial As(III) removal from the aqueous solution. The removal communities in the alkaline contaminated soil (Sun et al. mechanisms involved partial As(III) adsorption and the 2021c). The combination of biochar with ferrous sulfate oxidation of As(III) to As(V) by MnO . The generated and pig manure is also reported to effectively immobilize As(V) could be removed by forming MnHAsO4⋅H2O Cd and reduce Cd uptake by wheat in the alkaline con- precipitation with Mn(II), complexation with Mn-OH taminated soil (Chen et al. 2021b). groups on the MnO surface, and O-containing func- tional groups on the biochar surface (Fig. 3c) (Cuong 3.4.2 Bio char‑based catalyst for biofuel production et al. 2021). Nano zero-valent iron (nZVI) modified The employment of biochar as heterogeneous catalysts biochar has been proven to be a promising remediation for biofuel production gained much attention in 2021. material for Cr removal (Zhou et al. 2021b; Zhuang et al. Biochar-based catalyst possesses sustainable feedstock 2021). For instance, Zhuang et al. (2021) synthesized availability, abundant surface functional groups and inor- sulfidated nZVI-supported biochar for Cr(VI) removal. ganic species, a hierarchical and well-developed porous The porous structure of biochar and sulfidated nZVI structure, and tunable surface functionality (Chi et al. dispersed on biochar surface could provide adsorption 2021; Low and Yee 2021). sites for Cr(VI). Then, Fe and generated Fe(II) acted as Using biochar-based heterogeneous catalysts for bio- electron donors to reduce Cr(VI). The generated Cr(III) diesel production is still a great research hotspot in 2021. would form co-precipitation with Fe(III) and be removed The presence of biochar-based catalyst promoted the from the aqueous solution. transesterification or esterification of waste cooking oil, The employment of biochar in reducing the bio - vegetable oil, animal oil, or fats (Chi et al. 2021; Cho et al. availability and phytotoxicity of toxic metals in soil has 2021; Low and Yee 2021). Swine manure-derived biochar attracted growing interest. However, pristine biochar has could act as an alkaline catalyst for the transesterification a limited effect on the remediation of toxic metal-con - lipid fraction extracted from the swine manure (Fig. 4a). taminated soils (Arabi et al. 2021; El-Naggar et al. 2021). A high yield of biodiesel (≥ 94%) was achieved with the It was found that biochar, especially rice straw-derived application of the catalyst (Cho et al. 2021). A maximum biochar application contributed to the reduction of As(V) biodiesel yield of 96.4% was obtained with methanol: oil to As(III), which increased the potential mobility of As ratio of 9:1 at 70 °C for 2 h using K CO + Cu(NO ) - 2 3 3 2 in soils (El-Naggar et al. 2021). Heat treatment tempera- treated hydrochar-derived catalyst (Fig. 4b) (Abdullah ture (HTT) is a predominant parameter influencing As et al. 2021). Likewise, Nazir et al. (2021) obtained a bio- mobility in soil. A meta-analysis indicated that biochar diesel yield of 89.19% at 60 °C for 15 min in the transes- produced under low HTT (≤ 450 °C) did not affect As terification of oil with methanol at a molar ratio of 1:18, mobility in soil, but high-temperature biochar (> 450 °C) catalyzed by H SO -modified biochar. Moreover, the 2 4 exhibited high As mobilization in soil (Arabi et al. 2021). biochar-based catalyst exhibited high reusability and sta- Fe-modified biochar exhibited efficient removal of As bility with a minor loss in its activity after being reused from soil and reduced its mobility in paddy soil (Wen for six cycles. Microwave-assisted biofuel production has et al. 2021). Similarly, Fe/Al/Zn (hydr)oxides modified recently gained great attention due to the advantages of biochar significantly reduced As uptake by arugula (Sun microwave heating (i.e., quick heating rate and energy et al. 2021b). More importantly, Fe-modified biochar efficiency) (Nazir et al. 2021; Zailan et al. 2021). could be considered an efficient remediation material Syngas, produced through biomass gasification, usu - for moderately and highly Cd- and As-co-contaminated ally consists of H , CO, C H, CO , and other hydrocar- 2 4 2 farmland (Wen et al. 2021; Yang et al. 2021a, b). bons such as bio-oil or tar (Low and Yee 2021; Sun et al. 2021a). The addition of biochar as a catalyst for syngas Wu et al. Biochar (2023) 5:6 Page 8 of 21 Fig. 4 Employment of biochar as heterogeneous catalysts for biofuel production. Biodiesel production through biochar-catalyzed transesterification of lipid fraction extracted from the swine manure (a) (Cho et al. 2021); Biodiesel production with K CO + Cu(NO ) -treated 2 3 3 2 hydrochar as a catalyst (b) (Abdullah et al. 2021) production and upgrading has been largely reported a catalytic effect on tar removal (Feng et al. 2021a ; Hu recently. Biochar-based catalysts have several advan- et al. 2021a; Liu et al. 2021c). By using walnut shell bio- tages in tar reforming and syngas cleaning: (1) the highly char with high contents of K O and CaO as a catalyst, the porous structure and large SSA are conducive to prolong- conversion efficiency of tar and H -rich gas production ing the retention time of the reactants and improving increased (Mazhkoo et al. 2021). Anniwaer et al. (2021) their conversion efficiency; (2) the abundant functional reported that biochar derived from Japanese cedarwood groups favor the reforming of volatiles from biomass heat possessed a highly porous structure and contained high treatment; and (3) the alkali and alkali earth metals and contents of alkali and alkaline earth metals, which might other inorganic species present in biochar can provide provide high catalytic activity for tar reforming. In this Wu et al. Biochar (2023) 5:6 Page 9 of 21 study, 99% of the remaining tar was cracked/reformed, carbon sequestration potential of biochar produced from yielding H -rich syngas. Metal-supported biochar exhib- various crop residues. It was found that over 920 kg CO 2 2e ited excellent catalytic activity in syngas upgrading. Hu t- (CO -equivalent) sequestration could be achieved in et al. (2021a) loaded Fe–Ni on pine wood biochar as a China, demonstrating great carbon sequestration poten- catalyst for syngas production; their results indicated tial through biochar that was incorporated into soil that Fe–Ni-supported biochar significantly increased H / (Fig. 5a). Similarly, Leppäkoski et al. (2021) calculated the CO ratio to 1.97 in syngas. In addition, Fe–Ni-supported carbon footprint of willow biochar production and appli- biochar still exhibited excellent catalytic activity after five cation in marginal lands by conducting a cradle-to-grave times of reuse. Similarly, an H concentration of 67.35% LCA. Their results found that the carbon footprint of −1 in syngas and a gasification efficiency of 96.93% could willow biochar was − 1875 kg CO t , in which carbon 2e −1 achieve by using Fe/Ca/Al-loaded biochar as a catalyst sequestration (− 1704 kg CO t ) dominated the carbon 2e (Hu et al. 2021b). In short, engineered biochar has tre- footprint. It is estimated that 63 − 82% of initial carbon mendous promise as a catalyst in syngas upgrading. in biochar was stably sequestered in soil after 100 years of using a greenhouse gas inventory model (Woolf et al. 3.4.3 Biochar for global climate change mitigation 2021). These results indicate the long-term sequestration Biochar production and amendment have been increas- of carbon by biochar storage in soil. ingly adopted to mitigate global climate change (Woolf Biochar also has a high potential for CO capture and et al. 2021). China’s rising influence on global climate storage (Shafawi et al. 2021). It has been reported that change mitigation advances biochar’s role in climate gov- a large surface area, high micro-porosity, and abundant ernance. The functionality of biochar to achieve carbon– mineral contents of biochar contributed to C O capture neutral goals is mainly through carbon sequestration and (Feng et al. 2021b; Shafawi et al. 2021). To highlight, emission reduction (Cao et al. 2021; Nan et al. 2021). The N-doped biochar showed a superior C O uptake capac- high content of aromatic carbon within biochar is the ity. The N-doped biochar prepared by urea phosphate basis of its carbon sequestration benefits (Xu et al. 2021). impregnation-pyrolysis had a superior CO adsorption −1 Biochar storage in soil via agricultural soil management capacity of 1.34 mmol g (Ma et al. 2021). The excel - can also realize carbon sequestration (Guenet et al. 2021). lent adsorption property of CO was attributed to the Therefore, converting biomass waste into biochar and enhanced microporous structure and various N-contain- storing the produced biochar in soil has excellent poten- ing functional groups on the biochar (Fig. 5b). A related tial for carbon sequestration. Yang et al. (2021d) used a study by Feng et al. (2021b) showed that NH ·H O 3 2 life cycle assessment at the country level to evaluate the activation was beneficial to forming micropores and Fig. 5 Country-level potential of carbon sequestration for biochar implementation (a) (Yang et al. 2021d); The removal mechanisms of C O by N-doped biochar (b) (Ma et al. 2021); The removal mechanisms of CO by NH ·H O-activated biochar (c) (Feng et al. 2021b); The mitigation 2 3 2 mechanisms of N O Emission by biochar at the cellular level (d) (Zhang et al. 2021 g) 2 Wu et al. Biochar (2023) 5:6 Page 10 of 21 introducing N-containing functional groups on biochar resulted from the decrease in nitrifier denitrification (by surfaces. The N-containing functional groups dominated 74%) and heterotrophic denitrification (by 58%) (Zhang the adsorption of CO (Fig. 5c). et al. 2021e). The N stable isotope tracing techniques Nitrous oxide (N O) has a global warming potential may help to fully understand N O emission mitigation 2 2 of 298 times that of C O , which plays an essential role mechanisms by biochar. in global warming (Zhang et al. 2021g). Excessive appli- Biochar has been well-reported to mitigate C H emis- cation of N fertilization and low N use efficiency are sions from paddy soil (Dong et al. 2021; Jiang et al. 2021; the primary sources of soil N O emissions (Reay et al. Qi et al. 2021b). The mechanisms of CH emission miti- 2 4 2012; Tian et al. 2019). Numerous studies have demon- gation by biochar mainly included: (i) biochar inhibited strated the effectiveness of biochar in mitigating N O the activity and abundance of methanogens but stimu- emissions (Deng et al. 2021a; Jiang et al. 2021; Shin et al. lated those of methanotrophs (Nan et al. 2021); (ii) bio- 2021). Lower N O emissions involved several mecha- char increased the adsorption of CH in soil (Zhao et al. 2 4 nisms, which included abiotic N retention mechanisms 2021a). Contrastingly, other studies have found that bio- and microbial N immobilization mechanisms (Guenet char amendment increased CH emissions from paddy et al. 2021; Lehmann et al. 2021; Liao et al. 2021). The soils (Cao et al. 2021; Yang et al. 2021h). The unintended promoted microbial reduction of N O to N by biochar consequence of an increase in CH emissions may result 2 2 4 amendment was believed to be a permanent mitigation from the improved aeration condition after biochar benefit as the transformation could not be reversed, and application (Cao et al. 2021). Moreover, biochar’s geocon- N had left the soil system. The microbial reduction of ductor function could directly transfer electrons to meth- N O to N could result from an increased expression of anogens, stimulating CH production (Yang et al. 2021h). 2 2 4 denitrification-associated functional genes (e.g., nosZ, The contradictory studies could be due to the differences nirK, nirK, and narG) (Deng et al. 2021a; Liao et al. 2021; in biochar and soil properties (Malyan et al. 2021; Yang Wu et al. 2021b). Additionally, biochar could act as an et al. 2021h). Based on the discussions, biochar has great electron shuttle to facilitate electron transfer and N O potential in mitigating C H emissions from paddy soils. 2 4 reduction through biochar’s O-containing functional However, scientific soil management and proper biochar groups and carbon matrices (Yuan et al. 2021; Zhao et al. are necessary to avoid the unintended consequence of 2021c). It is important to note that the positive effects increased CH production. of biochar on denitrification metabolism could be elu - cidated at the cellular level by integrating physiological 3.4.4 Bio char for salinity and drought stress amelioration and multi-omics (proteomic and metabolomics) analy- Biochar has the potential to enhance salinity tolerance to ses (Zhang et al. 2021g). It was observed that biochar plants by improving soil physical (porosity, water hold- could directly modulate carbon metabolism and allocate ing capacity, hydraulic conductivity, pH, SOC), chemi- the produced reducing power, thereby promoting N O cal (Na bioavailability, cation–anion exchange capacity, reduction (Fig. 5d). N stable isotope tracing techniques nutrients, enzymatic activities), and biological (microbial have been widely used to gain insight into N O emissions activities, symbiotic N -fixation) properties (Farhangi- 2 2 in biochar-amended soils (Craswell et al. 2021). Zhang Abriz and Ghassemi-Golezani 2021; Singh et al. 2021). 15 18 et al. (2021c) utilized a dual isotope ( N– O) labeling Incorporating biochar in salt-affected soil improved 2+ 2+ + technique to differentiate the contribution of nitrifier soluble cation (Ca, Mg ) contents but decreased N a nitrification, nitrifier denitrification, nitrification-coupled concentration through its high sorption capacity. The 2+ + denitrification, and heterotrophic denitrification to soil higher Ca concentration and lower Na concentration N O emissions amended with biochar. Their results indi - facilitated K and P uptake by plants and thus promoted cated that biochar reduced N O emissions derived from plant productivity (Zhou et al. 2021d). It was also found nitrifier denitrification by 45–94%, nitrification-coupled that biochar amendment in saline soil increased photo- denitrification by 30–64%, and heterotrophic denitrifica - synthetic rate, leaf water content, stomatal conductance, tion by 35–46%. Biochar application for N O emission pigment contents, nutrient uptake, and root and shoot mitigation was due to the decrease of nitrite concen- growth (Cui et al. 2021a; Farhangi-Abriz and Ghassemi- tration while increasing N O reduction (Zhang et al. Golezani 2021; Liang et al. 2021c; Singh et al. 2021). In 2021c). A long-term field study also used a dual isotope some cases, toxic metal contamination and soil salinity 15 18 ( N– O) labeling technique to measure the effects of may cause a more serious environmental concern. Soil biochar on N O emissions; the results showed that bio- salinity may aggravate the stress caused by toxic metals char decreased N O emissions by 48% and 22% in acidic on plants (Azadi and Raiesi 2021; Shabbir et al. 2021). and alkaline soils, respectively. Lower N O emissions Biochar has been reported to mitigate the potential pres- sures of the co-occurrence of toxic metal contamination Wu et al. Biochar (2023) 5:6 Page 11 of 21 and salinity (Azadi and Raiesi 2021; Shabbir et al. 2021). Toxic metals and antibiotics/antibiotic resistance The salt stress could also be alleviated by supplement - genes (ARGs) are the main contaminants in composting ing biochar with other additives. In addition, combining products (Lu et al. 2021; Zhou et al. 2021a). It has been biochar with compost has increased plant productivity reported that biochar amendment aerobic composting (Liang et al. 2021b). is an efficient technology for reducing ARGs abundance Drought stress has been considered a major chal- in livestock manure and sewage sludge (Fu et al. 2021c; lenge for sustainable agricultural productivity (Man- Zhou et al. 2021a). Qiu et al. (2021) found that biochar soor et al. 2021). It has been discovered that biochar reduced the total abundance of ARGs by 17.6% dur- amendment improved the retention and availability ing sewage sludge composting. The analysis revealed of soil water, which led to enhanced stomatal con- that biochar reduced the abundance of bacterial patho- ductance, photosynthetic rate, and productivity gens such as Bacteroides and Pseudomonas. It is sug- under drought stress (Fu et al. 2021a; Kim et al. 2021; gested that change in the bacterial community by biochar Safahani Langeroodi et al. 2021). Biochar played a amendment dominated the reduction in the risk of ARGs positive role in nutrient supply and improved plant in manure/sludge composting (Mazhar et al. 2021; Qiu performance in the clay soil under drought conditions et al. 2021). Composting livestock manure or sewage (Mannan et al. 2021). The biochar amendment could sludge with biochar has also been reported to reduce also alleviate the oxidative damage of plants induced the mobility and bioavailability of toxic metals. In sheep by drought stress (Khan et al. 2021; Safahani Lan- manure composting, 10% biochar dose passivated copper geroodi et al. 2021). Overall, biochar is an efficient soil (Cu) and zinc (Zn) by 46.95% and 56.27%, respectively. amendment to ameliorate soil salinity/drought stress, Additionally, microbial diversity was improved, and improve soil functions, and promote plant productiv- Firmicutes was the dominant bacterial phylum in bio- ity in salt/drought-affected soil. char-based composting (Duan et al. 2021). In similar studies, biochar amendment had a positive impact on the diversity of toxic metal-resistant bacteria and toxic met- 3.4.5 Biochar amendment in composting als (Cu, Zn, Pb, Ni, Cr, As) passivation during livestock Composting is a promising technology to convert organic manure/sewage sludge composting (Liu et al. 2021b; waste into stable and humus-like products for use as Song et al. 2021; Zhang et al. 2021a). More importantly, organic fertilizer (Lu et al. 2021; Shan et al. 2021; Yin et al. reducing the bioavailability of toxic metals was responsi- 2021). Composting is a cost-effective way to manage agri - ble for lowering ARGs abundance (Qiu et al. 2021). cultural and breeding industry waste in China. However, No more than 10% biochar dose was recommended some issues such as greenhouse gases (CH, CO , and because excess biochar addition could cause severe water 4 2 N O) and odorous emissions (NH and H S), and nitro- loss and heat dissipation, thus negatively affecting the 2 3 2 gen loss during composting impede the development of composting process (Wang et al. 2021c). Moreover, the these practices (Yin et al. 2021; Zhou et al. 2021c). The cost of biochar may also be a limiting factor (Wang et al. functionality of biochar as a bulking agent for compost- 2021e). ing has been proven to be a promising strategy for solv- ing the environmental trade-offs of composting. Biochar 3.4.6 Bio char as additives in anaerobic digestion as additives could enhance the aeration rate and provide Anaerobic digestion (AD) is a biological treatment main shelters for microorganisms to enhance their activ- method to convert organic wastes into renewable biogas ity, thus improving the humification process and com - and biofertilizer and to sustain waste management posting performance as well as minimizing GHGs and (Ambaye et al. 2021; Su et al. 2021). Particularly, organic odor emissions. These capabilities are mainly attributed wet wastes, including livestock manure, food waste, and to its unique properties, including porous structure, large sewage sludge, are the most commonly used wastes for SSA, and abundant functional groups (Awasthi et al. AD. However, several key challenges persisted thor- 2021; Guo et al. 2021; Wang et al. 2021c). HHT is a criti- oughly, including low methane efficiency, operational cal parameter in assessing biochar’s function in mitigat- instability, unsatisfactory substrate degradation, and gen- ing GHGs emissions during composting (Yin et al. 2021). eration of toxic metabolic intermediates and gaseous pol- It is concluded that biochar pyrolyzed at high tempera- lutants (Zhang and Wang 2021). tures (500–900 °C) is more effective in mitigating CH Biochar has been identified as an effective additive to and N O emissions; in comparison, biochar produced at boost and improve AD performance (Qi et al. 2021c; low temperatures (< 500 °C) has a greater effect on reduc - Shi et al. 2021b; Sugiarto et al. 2021a). It could sig- ing NH emissions (Yin et al. 2021). nificantly assist in shortening the lag phase of organic Wu et al. Biochar (2023) 5:6 Page 12 of 21 Fig. 6 Proposed schematic of dark fermentative hydrogen enhanced with biochar (a) (Bu et al. 2021); The anaerobic digestion of waste activated sludge promoted by hydrochar and biochar (b) (Shi et al. 2021b) biodegradation, improving the production of methane volatile fatty acids)), and accelerated direct interspecies (CH ) and hydrogen (H ). Such improvement in AD electron transfer (DIET) (Ambaye et al. 2021; Bu et al. 4 2 process efficiency with biochar amendment could be 2021; Qi et al. 2021c). Bu et al. (2021) reported that bio- attributed to buffering capacity, adsorption of inhibi - char significantly boosted H production from pretreated tory substances (e.g., ammonia nitrogen (NH -N) and sugarcane bagasse by implementing efficient enrichment 4 Wu et al. Biochar (2023) 5:6 Page 13 of 21 Table 2 Biochar performance in anaerobic digestion (AD) for the treatment of organic wet wastes Feedstock HTT (°C) Substrate in AD Dosage Performance References −1 Wood chip 700, 800, 900 Seed sludge 12 g L 900 °C HTT enhanced Qi et al. (2021d) specific CH production to −1 742 mL g ethanol Fenton sludge 200, 400, 600, 800 Seed sludge + feeding – 400 °C HTT increased CH Wang et al. (2021b) substrate production by 38.1% −1 Pine sawdust 650, 900 Food waste 15 g L 900 °C HTT increased Sugiarto et al. (2021a) cumulative CH produc- tion by 46.9% −1 Corn straw 300, 400, 500 Kitchen waste 10 g L 400 °C HTT enhanced CH Wang et al. (2021a) production rate by 152% −1 Enhanced accumula- Zhang et al. (2021b) Horticultural waste < 500 Seed sludge + food waste 50 g L tive CH production to −1 126.7 mL g volatile solid −1 RIce husk 550 Corn stover + chicken 10 g L Enhanced specific CH Yu et al. (2021) manure production by 18.5% Waste apple tree branch 550 Potato pulp waste + dairy 2% TS content Potato pulp waste/ Chen et al. (2021a) manure dairy manure enhanced maximum CH yield of −1 200 mL g TS Rice husk 300 Swine manure5% TS Increased accumulative Yang et al. (2021f ) waste CH yield by 23.6% −1 Waste wood pellet 800 Food waste 25 g L Enhanced the ultimate Cui et al. (2021b) accumulative CH yield by 214% −1 Corn stover 300, 400, 500 Waste activated sludge 1.0 g g 300 °C HTT increased the Shen et al. (2021) maximum CH production rate by 181.6% −1 Hickory wood chip 400, 900 Seed sludge 12 g L 900 °C HTT increased Qi et al. (2021c) specific CH production to 725 mL CH /g ethanol −1 Corn straw Hydrochar (260, 320); Waste activated sludge 10 g L Hydrochar (260 °C) Shi et al. (2021b) Biochar (500, 700) increased the CH meth- ane yield by 25.6% −1 Corn stover500, HNO -modification Food waste 10 g L Increased CH production Gao et al. (2021) 3 4 by 90% −1 Corn straw 550, nZVI-modification Sewage sludge + food 3.0 g g Increased the maximum Zhang and Wang (2021) waste CH production rate by 49.87% and colonization of functional bacteria and activating the increased DIET between syntrophic microorganisms extracellular electron transfer between functional bac- and the activity of microorganisms (Qi et al. 2021a). Bio- teria (Fig. 6a). Moreover, biochar amendment could also char could advance MPs removal in AD through direct advance the removal of key contaminants during the AD adsorption. In addition, biochar amendment could pro- treatment of organic wet wastes, such as heavy metals, mote microbial activity in AD, thus enhancing the bio- antibiotics, polycyclic aromatic hydrocarbons (PAHs), degradation of MPs (Qi et al. 2021d). Table 2 summarizes and microplastics (MPs). Biochar has been considered a the effects of biochar on AD performance. passivator to reduce the bioavailability of heavy metals The mineral contents (e.g., Fe, Ca, K, Na) present in by electrostatic adsorption, physical adsorption, compl- biochar play a crucial role in promoting C H and hydro- exation, precipitation, and redox effect in AD (Qi et al. gen production. It was reported that the addition of 2021a). Adsorption and promotion of biodegradation leached biochar in the AD system resulted in lower C H are critical pathways for removing antibiotics with bio- and hydrogen production compared to unleached bio- char amendment in AD (Cheng et al. 2021). Biochar has char treatment (Sugiarto et al. 2021b). The role of iron high-effective performance in promoting the biodegrada - (Fe) within biochar in boosting C H production in the tion of PAHs in AD. The enhancement was attributed to AD system was largely reported in 2021. Wang et al. Wu et al. Biochar (2023) 5:6 Page 14 of 21 (2021b) showed that magnetite-contained biochar pre- resulting in the accumulation of toxic metabolic inter- pared from Fenton sludge pyrolyzed at 400 °C signifi - mediates that could delay methane production (Ambaye cantly improved AD performance due to magnetite’s high et al. 2021). conductivity. Fe-modified biochar could act as a catalytic medium for substrate hydrolysis and accelerating metha-4 Conclusions and perspectives nogenesis. The generated Fe oxides on the biochar sur - CiteSpace-based scientometric analysis was used to face contributed to the interspecies electron transfer in analyze the research trends and hotspots in the biochar syntrophic metabolism (Deng et al. 2021b). Therefore, field based on 5535 publications collected from WoS introducing Fe oxides into biochar has great potential to core collection in 2021. The number of publications has improve AD performance. expanded dramatically in 2021 and the growth trend may Hydrochar has also been demonstrated to enhance AD continue. China and USA were pioneers in this field. The performance. He et al. (2021) showed that hydrochar keyword clustering analysis indicated that “Biochar for promoted methane production rates by 36.4–237% in toxic metal immobilization”, “Biochar-based catalyst for AD of organic wastes via DIET mediated through surface biofuel production”, “Biochar for global climate change oxygen-containing functional groups. Hydrochar pre- mitigation”, “Biochar for salinity and drought stress ame- pared at lower temperatures had greater surface oxygen- lioration”, “Biochar amendment in composting”, and containing functional groups, which were related to the “Biochar as additives in anaerobic digestion” were the facilitated DIET. Similarly, Shi et al. (2021a) found that main research trends and hotspots in biochar research hydrochar increased the methane yield and production in 2021. The employment of biochar as heterogeneous rate by 31.4% and 30.8%, respectively. Genome-centric catalysts for biofuel production became the focus of bio- metatranscriptomics analysis revealed that hydrochar char research in 2021. Biochar’s applications for heavy behaved as an electron shuttle to promote DIET between metal immobilization, global climate mitigation, salin- Syntrophomonas sp. FDU0164 and Methanosarcina sp. ity and drought stress amelioration, biofuel production, FDU0106. It should be noted that hydrochar exhibited and anaerobic digestion promotion represent sustainable a superior ability to promote methane yield and produc- growing topics in 2021. However, bioremediation using tion rate than biochar. Hydrochar had greater surface functional bacteria immobilized on biochar and biochar- oxygen-containing functional groups, which were related assisted advanced oxidation process were well-studied to facilitated DIET and enhanced methane yield and pro- but with less frequency in 2021 than in 2020. In short, the duction rate. The metabolomic analysis also showed that present review provides a comprehensive overview of the the alterations of metabolites associated with fatty acids research and the evolution of research hotspots in bio- and amino acids metabolism induced by hydrochar were char research. stronger than those of biochar (Fig. 6b) (Shi et al. 2021b). Although biochar has multifunctional applications in The addition of biochar in the anaerobic co-digestion agriculture, environment, and energy, its potential eco- of food waste or livestock manure with sewage sludge has logical risks, and long-term safety and implications are recently aroused considerable attention. It has highly syn- of great concern. Previous studies have confirmed that ergistic effects on hydrolysis acidification and methane biochar contained several toxic components, includ- production compared with digestion alone (Johnravin- ing polycyclic aromatic hydrocarbon (PAHs), volatile dar et al. 2021; Liang et al. 2021a; Zhang et al. 2021d). organic compounds (VOCs), polychlorinated dibenzo- A recent study indicated that the anaerobic co-digestion p-dioxins (PCDDs), persistent free radicals (PFRs), toxic of pig manure with municipal sludge enhanced methane metals, and water-soluble organic compounds (WSOCs) yield by 83.0–136.5% and 31.3–68.0% at mesophilic and (Brtnicky et al. 2021; Godlewska et al. 2021). The detri - thermophilic temperatures, respectively (Zhang et al. mental effects of these harmful substances were corre - 2021d). Notably, certain disadvantages of incompatible lated with their bioavailable fraction, which depended inoculum-to-substrate ratio, excessive acidification, and on the feedstock and preparation methods (Godlewska organic overloading may lead to the inhibition of biogas et al. 2021; Wang et al. 2019). PAHs are mostly formed production (Liang et al. 2021a). during the incomplete combustion of biowaste (Wang It is noted that biochar/hydrochar addition, exceed- et al. 2017). It is suggested that biochar produced by ing a certain amount, could negatively affect AD perfor - slow pyrolysis had lower PAHs than by fast pyrolysis mance. In some cases, the negative effect was due to the (Wang et al. 2017). Biochar produced under medium high sorption capacities of biochar, which might reduce temperatures (400–600 °C) often contained higher con- the contact between microbes and substrates. Another tents of bioavailable PAHs. In general, the majority of potential explanation is that a high dose of biochar accel- biochars contained a low bioavailable fraction of PAHs erated the hydrolysis and acidogenesis-acetogenesis, (Tomczyk et al. 2020). PCDD/Fs are mainly formed on Wu et al. Biochar (2023) 5:6 Page 15 of 21 the surface of biochar during the thermal treatment of contaminants are mainly originated from feedstock feedstock, especially food waste (El-Naggar et al. 2019). rich in heavy metals, especially sewage sludge, animal It is reported that low temperatures (200–400 °C) and manure, and plants grown on heavy metal-contami- short residence time favored the formation of PCDDs nated soils. The conversion of biomass abundant with on biochar surface (Lyu et al. 2016). It is accepted heavy metals to biochar is considered a promising that PCDDs always existed in low quantities in bio- method for safely disposing of biomass and significantly char and posed a relatively marginal risk to organisms reducing the bioavailability and leaching of heavy met- (Godlewska et al. 2021; Weidemann et al. 2018). VOCs als (Devi and Saroha 2014; Wang et al. 2019; Zhang et al. are mainly formed on biochar surfaces or/and inside 2020). The contents of certain heavy metals increased biochar pores by the thermal decomposition of bio- with increasing HTTs, which may be attributed mainly mass. The contents of VOCs in biochar decreased with to the ‘concentration effect’ resulting from a decrease increasing HTTs (Buss et al. 2015). Numerous stud- in biochar yield (Zhang et al. 2020). The bioavailabil - ies have reported the stable and large molecular PFRs ity of heavy metals depends on heavy metal contents in biochar during the biomass carbonization process in raw biomass and the transformation and dissolution (Liao et al. 2014; Lieke et al. 2018; Yang et al. 2016; Zhen of heavy metals in biochar (Godlewska et al. 2021). The et al. 2021). It was found that the electron paramagnetic release of water-soluble organic compounds (WSOCs) resonance (EPR) signals in lignin-derived biochar were from hydrochar was higher than from biochar. With the higher than those in cellulose-derived biochar. The EPR increase in HTT, the contents of phenols and organic signals increased with increasing HTT for the major- acids in WSOCs increased (Hao et al. 2018). ity of biochar (Liao et al. 2014). Besides organic com- Despite the ecologically acceptable levels of these pounds, heavy metals (e.g., Cd, Cu, and Pb) in biochar harmful substances for most biochar, it may pose an are highly concerned with their toxicity. Heavy metal ecological risk to soil biota (Fig. 7). For example, it is Fig. 7 Potential risks associated with biochar application to soils Wu et al. Biochar (2023) 5:6 Page 16 of 21 Received: 21 September 2022 Revised: 26 December 2022 Accepted: 2 revealed that the attendance of some contaminants (e.g., January 2023 PAHs, cresols, methylated phenols) in biochar might cause direct toxicity to soil microorganisms (Oleszczuk and Koltowski 2018; Yang et al. 2019), for example, sig- nificant germination inhibition, plasma membrane dis - References ruption, and plant growth retardation (Liao et al. 2014). 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J Clean Prod 291:125947. https:// Supplementary Information doi. org/ 10. 1016/j. jclep ro. 2021. 125947 The online version contains supplementary material available at https:// doi. Azadi N, Raiesi F (2021) Salinity-induced changes in cadmium availability affect org/ 10. 1007/ s42773- 023- 00204-2. soil microbial and biochemical functions: mitigating role of biochar. Chemosphere 274:129924. https:// doi. org/ 10. 1016/j. chemo sphere. 2021. 129924 Additional file 1. Table S1 The top 10 most productive countries in Brtnicky M, Datta R, Holatko J, Bielska L, Gusiatin ZM, Kucerik J, Hammer- biochararea in 2021. Table S2 The top 10 institutions in the field of schmiedt T, Danish S, Radziemska M, Mravcova L, Fahad S, Kintl A, biocharresearch in 2021. Fig. S1 The number of published documents on Sudoma M, Ahmed N et al (2021) A critical review of the possible biocharresearch. Fig. S2 Countries performing biochar research in 2021. adverse effects of biochar in the soil environment. Sci Total Environ 796:148756. https:// doi. org/ 10. 1016/j. scito tenv. 2021. 148756 Acknowledgements Bu J, Wei HL, Wang Y T, Cheng JR, Zhu MJ (2021) Biochar boosts dark fermenta- Not applicable. tive H2 production from sugarcane bagasse by selective enrichment/ colonization of functional bacteria and enhancing extracellular electron Author contributions transfer. Water Res 202:117440. https:// doi. org/ 10. 1016/j. watres. 2021. PW: Investigation, writing-original draft and editing, funding acquisition; BS: 117440 Writing-review and editing; HW: Review and editing; ZJ: Investigation; YW: Buss W (2021) Pyrolysis solves the issue of organic contaminants in sewage Investigation, review and editing, funding acquisition; WC: Investigation, sludge while retaining carbon—making the case for sewage sludge review and editing. All authors read and approved the final manuscript. treatment via pyrolysis. ACS Sustain Chem Eng 9:10048–10053. https:// doi. org/ 10. 1021/ acssu schem eng. 1c036 51 Funding Buss W, Masek O, Graham M, Wust D (2015) Inherent organic compounds This work was supported by the National Natural Science Foundation of China in biochar—their content, composition and potential toxic effects. J (Project No. 42225701, 42007355). Environ Manage 156:150–157. https:// doi. org/ 10. 1016/j. jenvm an. 2015. 03. 035 Availability of data and materials Cao Y, Shan Y, Wu P, Zhang P, Zhang Z, Zhao F, Zhu T (2021) Mitigating the All data generated during the current study are available from the correspond- global warming potential of rice paddy fields by straw and straw- ing author on reasonable request. derived biochar amendments. Geoderma 396:115081. https:// doi. org/ 10. 1016/j. geode rma. 2021. 115081 Cha JS, Park SH, Jung S-C, Ryu C, Jeon J-K, Shin M-C, Park Y-K (2016) Production Declarations and utilization of biochar: a review. 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Biochar – Springer Journals
Published: Jan 18, 2023
Keywords: Bibliometric analysis; Citespace; Research hotspots; Toxic metal immobilization; Sustainable application
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